In the history of gas-turbine engines, significant work has been done and success had with respect to high-pressure-ratio (HPR) compressors and high-temperature air-cooled turbines. These engines generally do not employ pressure heat exchangers to capture heat from the turbine exhaust, are efficient only when close to full load and employ ratios at well over 10:1. In large engine sizes and at full-load operation, as is common for aerospace use, the engines are extremely effective. Adapting this technology to use in the automobile market, however, yielded difficulties since small compressors could not operate at a maximum compression ratio of 10:1 without suffering penalizing losses in efficiency due to the very small high-pressure passages required and the consequent relatively large blade-tip clearances.
Another problem suffered by those attempting to employ HPR turbines for the automotive market is that automobiles generally run (almost exclusively) at part-load. As was stated hereinabove, the maximum efficiency of HPR engines is obtained at full load. Unfortunately efficiency cannot be maintained by HPR turbines at part-load.
Upon such discovery, engineers turned to lower pressure ratio (LPR) engines (pressure ratios on the order of 4:1) with a heat exchanger. The pioneering exchangers/regenerators were constructed of alternating straight and corrugated strips of stainless steel wound around a mandrel and brazed. The regenerator functioned as intended; however, it did not lend itself to mass production and was far too expensive to produce to compete effectively with spark-ignition engines of the day. Although this initial attempt did not result in an economically feasible construction, research interest did not wane.
Ceramic regenerators appeared to provide desirable characteristics at low enough cost. Unfortunately however, it was found that sulfur and sodium contamination from exhaust and road salt, respectively, caused failures of the then popular lithium-aluminum-silicate ceramic material preferentially used in the heat exchangers. Aluminum silicate and magnesium aluminum silicate were also experimented with and gave superior results. Commonly the regenerators were monolithic and so when damaged by the contaminants noted above or by other means the entire regenerator had to be replaced at not insignificant cost. Unless regenerators could be more reliable and less costly the turbine engine might never be adopted.
Ceramic regenerators of this type also suffered from seal wear. The seals employed had to rub against the outer surface of the disk and wore quickly. Such problems did not reduce interest in ceramic regenerators, however, because the preferred turbine-rotor-inlet temperatures for small turbines had increased to about 1650 K. This temperature coupled with a relatively low cycle pressure ratio gives a turbine expander exit temperatures beyond the melting point of metal heat exchangers. Since at such temperatures, ceramics are the only economically feasible alternative, development continued with respect to the material, manufacturing methods and seals.
Manufacturing of some of the first ceramic regenerators was accomplished by employing the older wrapped-mandrel approach with a ceramic-soaked paper material. Unfortunately however, such methods are neither low cost nor extremely high performance. Performance suffered due to non-optimum passage shapes and spacially non-uniform resistance characteristics. To alleviate this problem, methods of extruding regenerators were developed.
While extrusion is lower in manufacturing cost and reliably provides spacially uniform flow resistance, due to uniform passage size and shape, the extruded regenerators are limited in possible outer dimension simply due to limitations inherent in extruding large objects (300 mm is the largest diameter known to the inventor). Extruding large objects necessitates high pressure even with relatively low-viscosity materials. Ceramic materials found to be preferred in the manufacture of rotary regenerators are not of low viscosity and, therefore, require even higher pressures. This is problematic because of both of rapid frictional wear of the dies and a high incidence of breakage thereof due to stress. Therefore, the extrusion method for forming rotary regenerators of much over 300 mm in diameter (in one piece) has not proved viable. Moreover, if the regenerator were to experience a failure of a part thereof, repair is not an option. The regenerator would need to be replaced. This increases expense to the owner.
Furthermore, single-piece extruded regenerators are limited to a maximum diameter of about 300 millimeters and, therefore, necessarily limit the turbine size to engines below 100 kw or will result in significant compromise in the design parameters of larger engines. Other plaguing problems surrounding a viable regenerator and therefore a workable and economical automotive turbine engine are the seals.
All of the seals known to the inventor to be employed by those working toward an automotive turbine are rubbing seals. By definition, rubbing seals do rub, and consequently wear. Moreover, to reduce the rate of wear of these seals they are utilized in such a manner that they do not clamp against the regenerator surface with a significant enough force to be very pressure containing. Lower clamping force reduces friction which is beneficial in the fight against wear. The drawback of this, of course, is that the seals generally will not hold a large pressure gradient across the seal. The pressure ratio of the machine then is limited and efficiency suffers. Add to this that even with the lower clamping pressure the seals wear at too speedy a rate for competing in the spark-ignition or compression-ignition markets, and the turbine fails to be a desirable option again. In fact, actual leakage rates across these seals frequently increases to over 15% of the compressor air flow in a very short period of time. Loss in overall engine thermal efficiency is approximately at a one-to-one ratio for each point of leakage increase. Advances calculated and proven to effect more efficient regeneration without high leakage are needed before an automotive turbine might become a reality.
On another front, most prototype automotive turbines have employed a single-stage centrifugal (radial) compressor driven by a single-stage axial turbine, followed by a second independent single-stage axial power turbine exhausting into two parallel rotary regenerators. Drawbacks that have plagued development of these engines are that the regenerators discussed above had high leakage and high manufacturing costs. The turbine engines in general had poor peak-load economy and high manufacturing cost. In fact, the cost of manufacture has been several times greater than for spark-ignition engines.
Creating viable engines optimally suited for automotive use requires minimization of the above identified drawbacks and shortfalls of prior art engines.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the engine and regenerator of the invention.
Alleviation or eradication of the regeneration problems for automotive turbine engines of the past is accomplished by providing a multiple-sectored or more properly, multiple-sectored regenerator assembleable into one or two regenerator disks. In this manner a large disk may be assembled by extrusion methods since each sector, which is much smaller by itself, is individually extruded. The matrix is also extrudable in small assembleable sectors. The regenerator sectors are either extruded in a shell and matrix combination or in a separate manner with the matrix sized to fit within the shell. Alternatively the matrix elements may be stamped. Alternatively, the matrix may be of foamed ceramic, sintered spheres, wires or rods or other materials having flow through capability in order to function as a rotary regenerator material as described herein or otherwise known. Significant advantages both in cost and function of the disk are realized.
Engine design having multiple compressor stages and multiple turbine stages at a low pressure ratio in combination with the regenerator described herein, is in a different direction from the prior art. The invention employs an axial-flow-turbine design, at a low pressure ratio which, of course, also dictates a low peripheral speed of the rotors. Additional savings are realized in materials themselves and in the sizes or thicknesses of the components as a direct consequence of the lower pressure and speeds of the multistage engine of the invention.
The invention produces simultaneously higher thermal efficiencies, particularly at part load (the common condition of operation of engines in automobiles), reduces stresses on components thus increasing longevity, and allows the use of low-cost materials to lower the total initial cost of the engines.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.